Method and apparatus for the production of a thermoplastic container from a preform

ABSTRACT

A container is produced from a preform which is formed by expansion in the axial and/or radial direction of a hollow blank of originally substantially amorphous material. In the production, the circumference of the preform is reduced by heating of material in the preform body, at least one material portion of the container body being displaced, by means of a mechanical forming device (30), in a direction towards the centre of the preform for forming an outer bounding surface of the container comprising material portions whose distance to the centre axis of the container is less than the distance of corresponding material portions to the centre axis (26) of the preform. An apparatus for producing the container includes the mechanical forming device which, during reforming of the preform into the container, abuts against material in the preform. The mechanical forming device includes a forming portion (33) provided with channels for thermal medium and with a forming surface (37) disposed, under energy exchange with material in the preform, to displace material shrunk by heating of the material towards the centre axis of the preform.

The present invention relates to a method and an apparatus for producinga container in accordance with the preamble of the appended independentclaims.

There are needs in the art for a container of thermoplastic possessingsuperior configurational stability also at elevated temperatures. Inaddition, it is generally required that the container has at least onesubstantially planar side surface and, in particular, there is a needfor a container with, for example, substantially triangular, quadratic,rectangular or other substantially polygonal cross section. Finally, itis necessary that the container possess a superior capability towithstand both partial vacuum and excess pressure within the sealedcontainer, without any resultant troublesome change in shape.

A container possessing the properties disclosed in the foregoingparagraph is eminently suited for a multiplicity of fields of use, forinstance for the storage of foods. In particular in practicalapplications in which pasteurization, hot-filling or sterilizationoccur, it is necessary that the container, without changing shape,withstand the elevated temperatures involved. It will also be obvious tothe skilled reader that, in practical applications in which thecontainer has been filled with, for example, beverages containing, forinstance, carbon dioxide or nitrogen gas, high pressures occur in thesealed container both in connection with the filling operation andduring subsequent storage. It should particularly be observed that veryhigh pressures occur within a container which is filled with carbonatedbeverages when such a container is exposed to heating.

As one non-restrictive example of a thermoplastic material for theproduction of a container for the above-outlined purposes, mention mightbe made of polyethylene terephthal ate, hereinafter generallyabbreviated to PET. Like most other thermoplastic materials, PET ischaracterized int. al. in that it obtains, by mono and/or biaxialorientation, superior mechanical strength properties, that the materialis thermocrystallizable and that its barrier properties are generallysufficient for many practical applications within the food sector.

As has already been briefly disclosed above, the mono and/or biaxialorientation of the thermoplastic material entails that the materialobtains the contemplated superior mechanical strength properties, bywhich is primarily taken to mean a capacity to withstand, withoutappreciable configurational change, increased internal pressure in thoseparts of the container in which are included oriented material. In theproduction of the container, the point of departure is, as a rule, ablank of substantially amorphous material, the blank being expanded inconnection with production of the container. On the expansion, thethickness of the material is reduced at the same time as the material isoriented. Normally, only material in the walls of the container body isexpanded, the material being, as a rule, given a biaxial orientation.However, in certain practical applications, the material in the mouth oropening portion of the container is also expanded/oriented. On the otherhand, in most practical applications, the material in the bottom portionof the container is not expanded/ oriented, at least not in the centralregion of the bottom portion.

As a rule, the non-oriented material portions are thermally crystallizedso as to increase configurational stability at both low temperatures andelevated temperatures. In certain practical applications, thenon-oriented material portions are, in connection with the thermalcrystallization, exposed to external pressure forces (generallymechanical forces) in order to control the crystallization speed. Insuch instance, a combination of compression forces and temperatures isgenerally selected which entails that the material thickness undergoes acertain reduction, at least a reduction of a minimum of 1%. This willensure, on thermal crystallization by abutment against mechanicaldevices, also good contact between the mechanical devices and thematerial intended for crystallization. The increased crystallinityobtained as a result of the thermal crystallization, in combination withthe material thickness, entails the contemplated mechanical strengthproperties and the sought-for thermal stability for the container even in the non-expanded/oriented material regions.

Those portions in the container which consist of oriented material showa tendency to shrink at elevated temperatures. Naturally, this is amajor drawback when the container is employed for storing foods, sincethe containers are, in connection with the filling operation and/orduring subsequent handling, exposed to relatively high temperatures,i.e. temperatures which, in certain practical applications, approach150° C.

The present invention discloses a method and an apparatus for producingcontainers of thermoplastic material in which the above-disclosedrequirements are satisfied and the above-outlined problems are obviated.

The reduction of the circumference of the preform caused by heatingaccording to the invention entails a release of those tensions whichhave been embodied into the material in connection with orientation ofthe material. On renewed heating to a temperature corresponding to orslightly less than the temperature of the material in connection withrelease of the tensions, the material shows no tendency to shrink, forwhich reason the container retains its shape also in those portionswhich consist of oriented material.

According to one preferred embodiment of the present invention, thepreform is displaced through a passage formed by a mechanical devicewhich heats the plastic material and utilizes the shrinkage andworkability of the material at the elevated temperature in order, on thereduction of the circumference of the container body, to realizesubstantially planar side surfaces of the container body. Since,according to the invention, the passage is of smaller circumference thanthe outer circumference of the container body, all of those parts of thecontainer body which are to undergo circumferential reduction arebrought into reliable abutment against the mechanical device, theseparts thereby being heated to the desired elevated temperature. This isdetermined by the temperature of the abutment-forming surfaces of themechanical device. It will also be obvious that, because the orientedmaterial is relatively thin, the material will reach the elevatedtemperature already after a short abutment time.

The present invention will be described in greater detail herein below,with reference to a number of Drawing Figures, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-c show axial cross sections of a first embodiment of anapparatus for producing a container from a preform;

FIG. 2 is a cross section taken along the line II--II in FIG. 1c; and

FIG. 3 is an axial cross section corresponding to the cross section inFIG. 1a of an alternative embodiment.

FIG. 1a depicts a preform 20 of thermoplastic material with a mouthportion 21, a preform body 22 disposed between the mouth portion and thebottom portion, and a sealed bottom portion 23. The mouth portion isprovided with a mouth edge 25 which, in the embodiment of the preformshown in the Figure, defines an outwardly flared flange 27 of the mouthportion. The centre axis of the preform carries reference numeral 26.The preform body 22 has an outer, substantially cylindrical boundingdefinition 24 which, in all parts, is located more distally from thecentre axis 26 than any part of the mouth portion.

FIG. 1b shows the preform under reforming into a container 10 asillustrated in FIG. 1c. The container 10 has a mouth portion 11, acontainer body 12 and a sealed bottom portion 13. The mouth portionincludes a mouth edge 15 which, in the embodiment illustrated in FIG.1c, defines an outwardly flared flange 17 of the mouth portion. Thecentre axis of the container carries reference numeral 16 and the outerbounding definition of the container 12 has reference numeral 14. In theillustrated embodiment, the cross section of the container body issubstantially quadratic, but it will be obvious to a person skilled inthe art that the present invention is applicable for producing acontainer of any optional cross section, such as circular, elliptical,polygonal, etc, and also of embodiments in which parts of the boundingdefinition of the cross section are curved in order, in certainpractical applications, to form transitions between mutually adjacentside surfaces of the container.

FIGS. 1a-c also show a mechanical forming device 30 which includes aforming portion 33 forming a central passage 31 which, in theillustrated embodiment, is of substantially quadratic cross section (cf.FIG. 2). Channels 60 for thermal medium, preferably liquid, are providedin the forming portion 33. The forming portion is provided with anin-feed portion 32 and a discharge portion 34, the terms in-feed portionand discharge portion being related to that sequence in which thepreform is displaced in relation to the passage during the reforming ofthe preform into the container (cf. the description below). The in-feedportion includes a guide-in surface 35 and a contact surface 36 forpreheating material in the preform. The forming portion 33 includes aforming surface 37, and after-treatment surface 38 and a contact surface39 for cooling the formed container. It will be obvious to a personskilled in the art that, in one preferred embodiment, the mechanicalforming device is provided with thermal insulation which separates partsof the forming device which are set at different temperatures by thermalmedium.

The mechanical forming device also includes a press tool 40 which, byrelative displacement in relation to the forming portion 33, is moved toa position within the passage 31 and/or therethrough and thence back tothe starting position. The press tool is provided with a press head 41and, in the illustrated embodiment, with a rod 43 for connection todrive means (not shown on the Drawings). The press head is of aconfiguration adapted to the mouth portion of the preform, and therebyof the container, for generally substantially sealing abutment againstthe mouth portion during reforming of the preform into the container. Incertain embodiments, a channel 42 for pressure medium places, during thereforming process, the interior of the container in communication with apressure source (not shown on the Drawings). In certain practicalapplications, the mechanical forming device 30 also includes a supportsleeve 70 disposed in association with the in-feed portion 32 of theforming portion 33. The support sleeve has an inner configuration andcircumference corresponding to the cross section of the preform body inorder, on displacement of the preform into and through the formingportion 33, to support the preform exteriorly.

It will be apparent from the cross section illustrated in FIG. 2 thatthe forming portion 33 forms, in the illustrated embodiment, a passageof substantially quadratic cross section. In the transition betweenmutually adjacent side surfaces 14 of the inner definition of thepassage, the corners are rounded for forming curve abutment surfaces 64.In the Figure, the cross section of the preformed body is marked bybroken lines 65a, b, while the cross section of the container body ismarked by solid lines 66. The broken line 65a marks a preform whosecross section substantially corresponds to a circular periphery, ofwhich the curved abutment surfaces 64 at the corners form part, whilethe broken line 65b marks a preform of larger cross section.

In the embodiment illustrated in FIG. 3, the forming portion 33 isprovided with separate and mutually interconnected devices which form,first, the in-feed portion 32 of the forming portion, secondly, anintermediate portion 63 which includes the forming surface 37 andafter-treatment surface 38 of the forming portion, and thirdly thedischarge portion 34 with its contact surface 39 for cooling thematerial in the formed container. A coating or a layer 50a-c whichconsists of material of superior thermal conductivity and of lowfriction, for example copper or a copper alloy, is, as a rule, providedon the inside of the forming portion 33. Generally, the coating or layerforms all surfaces against which the preform abuts when it is urgedthrough the passage 31, i.e. the guide-in surface 35, the contactsurface 36 for preheating, the forming surface 37, the after-treatmentsurface 38 and the contact surface 39 for cooling. All of theabove-disclosed separate parts are provided with channels 60 for thermalmedium. A thermal insulation 61 is provided in certain embodimentsbetween the in-feed portion 32 and the intermediate portion 63 and, incertain embodiments, a thermal insulation 62 is also provided betweenthe intermediate portion 63 and the discharge portion 34. It will beobvious that, in certain embodiments, the channels for thermal mediumform closed systems for each respective part, which makes possibleadjustment of the temperature of each respective part to a predeterminedlevel. At least the discharge portion is set at a temperature deviatingfrom that of other parts, as a rule a lower temperature. Normally, thein-feed portion 32 is at a lower temperature than the intermediateportion 63.

When the apparatus according to any of the embodiments shown on theDrawings is reduced into practice, the preform 20 is placed in relationto the mechanical forming device 30, in a position in which the bottomportion 23 of the preform is located in the region of the in-feedportion 32 of the forming device. By relative displacement of thepreform and the forming portion 33, the bottom portion is moved intoabutment against the guide-in surface 35 of the in-feed portion,thereafter into contact against the contact surface 36 for preheating ofmaterial in the preform, thereafter into contact with the formingsurface 37 for reforming of the preform into a configuration (shape)corresponding to that of the container, and passes therefrom furtherpast the after-treatment surface 38 for possible supplementary releaseof tensions embodied into the plastic material, and thereafter, undercontinued abutment against the forming portion, past the contact surface39 for cooling in order, after completed displacement through thepassage 31, to depart from the forming portion 33 of the mechanicalforming device 30.

The displacement of the preform through the passage 31 and the reformingof the preform into the container realized in connection with thisdisplacement require that the preform be urged through the passage byexternal forces. This is achieved in that the press tool 40, duringabutment against the mouth portion of the preform, is displaced towardsthe forming portion 33 and through the passage 31 and, in such instance,urges the preform through the passage. It will hereby be ensured thatthe preform is brought into abutment against the hot forming surface 37of the forming portion 33 in all of those material regions of thepreform which are reformed during the displacement through the passage31 of the forming portion. It will be obvious that the preform is,during the reforming process, subjected to axially directed forces of amagnitude which may entail that the preform is deformed in that its wallis creased. In practical applications in which such a risk exists, theinterior of the container is, as a rule. pressurized using pressuremedium which is supplied via the channel 42, and also the support sleeve70 is, as a rule, disposed in order to contribute, by its outersupporting action, to minimizing the risk of crease formation. Inpractical applications in which the embodiment illustrated in FIG. 3 isemployed, the in-feed portion 32 generally constitutes a supportingmember corresponding to the support sleeve 70. The displacement of thepreform is adapted to the time which is required for heating of theplastic material so that this reaches a temperature suited for thereforming at the latest in connection with reforming of the preform.This temperature exceeds the glass transition temperature (Tg) of thematerial.

The rate of displacement of the preform through the passage is adaptedto the material properties of the thermoplastic material and to thesurface temperature of the contact surfaces of the forming portion, inorder, during each phase of the displacement, that the material willattain, by energy exchange with the forming portion 33, a temperaturesuited for each treatment stage. It is essential that all of those partsof the container body which undergo reforming reach at least thepredetermined minimum temperature for release of the tensions embodiedduring the orientation and reforming before these parts arrive at thecooling surface 39.

In one preferred practical application of the present invention in whichthe design of the forming portion 33 as illustrated in FIG. 3 isemployed, the rate of displacement of the preform is lower in theinitial stage (the temperature conditioning) than during its subsequentdisplacement. The axial length of the in-feed portion 32, and, thereby,the length of the contact surface 36 in the axial direction forpreheating is, in one preferred embodiment, selected so as substantiallyto correspond to the axial length of the essentially cylindrical portionof the container body. The lower rate of displacement of the preform inrelation to the forming portion during the first part of thedisplacement through the passage is, as a rule, selected when the axiallength of the in-feed portion is relatively short. It will hereby beensured that the material in all of those parts of the preform which areto undergo reforming will be heated to such an elevated temperature thatthe contemplated release of the tensions embodied in the material willbe achieved.

In certain practical applications, the axial length of the contactsurface 36 is greater than the axial length of the preform body. This isgenerally combined with an unchanged rate of displacement of the preformon its displacement through the passage 31 of the forming portion 33. Inother practical applications, the temperature of the in-feed portionvaries such that it is preferably highest most proximal the intermediateportion 63. By adapting the rate of displacement and surfacetemperatures of the forming portion 33 to one another and to thematerial thickness of those material portions which aretemperature-conditioned, the contemplated effect will be achievedemplying reforming of the preform, heating for releasing inner tensionsand cooling in connection with discharge of the formed container. Thedimensions of the inner bounding definition of the passage and themaximum temperature to which the material is heated are adapted to oneanother in order that, during the reforming, the circumference of thepreform body and the circumference of the container body, respectively,correspond to the inner circumference of the passage. In certainpractical applications, the rate of displacement of the container isreduced under passage of the contact surface 39 for cooling.

As has already been disclosed, the plastic material is, on reforming, ata temperature which exceeds the Tg of the material. In order to achievethis, the mechanical forming device is, in one preferred practicalapplication, at a temperature exceeding the Tg of the plastic materialby at least 20° C., preferably by at least 40° C. and generally by atleast 60° C.

By way of example of temperatures employed in reforming a preform ofPET, mention might be made of a temperature for the abutment surfaces ofthe intermediate portion 63 of at least 120° C., preferably at least140° C. and, in certain high temperature applications, at least 160° C.It has proved possible in practice to use temperatures for reforming thepreform of PET of as much as in the range of 180-200° C. The temperatureemployed is always higher--as a rule at least 5° C. higher--than thehighest temperature at which the material was adjusted in connectionwith undergoing the forming process or processes at which the amorphousblank was expanded into the preform. The temperature of the in-feedportion 32 is at least 10-15° C. lower than that of the intermediateportion 63, while the temperature of the discharge portion is, as arule, lower than the glass transition temperature (Tg) of the material.

The above-described technique discloses a dependable and reliabletechnique for reforming, by abutment against mechanical forming devices,a preform into a container under reduction of the circumference of thepreform and release of tensions embodied in the preform. There willhereby be achieved the contemplated temperature stability of thecontainer, a temperature stability which, on each occasion, is adaptedto the maximum temperature which the thermoplastic material in thecontainer may be expected to reach in the future use of the container.Furthermore, the elevated temperature is utilized in connection withreforming of the preform into the container for two different purposes,namely for softening the material and thereby making possible apermanent and temporally stable reforming and for releasing tensionsembodied in the material, thereby ensuring that the container does notshrink on being heated.

The above detailed description has referred to but a limited number ofembodiments of the present invention, but a person skilled in the artwill readily perceive that the present invention encompasses a largenumber of embodiments without departing from the spirit and scope of theappended claims.

I claim:
 1. A method for the production of a container (10) ofthermoplastic material, with a mouth portion (11), a container body(12), and a bottom portion (13) from a preform (20) with a mouth portion(21), a bottom portion (23) and a preform body (22), in which thepreform has been formed by expansion in the axial, and/or radialdirection of a hollow blank of originally substantially amorphousmaterial, comprising the following steps:forming the preform; heatingthe material in the preform body (12) by abutment against a mechanicaldevice (33) of a mechanical forming device (30) at elevated temperature,thereby reducing the circumference of the preform; displacing at leastone material portion of the preform body for formation of the container(10), through a passage (31) wholly or partly defined by the mechanicaldevice (33) in a direction towards the centre of the preform; forming anouter defining surface (14) of the container with material portionsbeing at a distance to the centre axis (16) of the container less thanthe distance of corresponding material portions to the centre axis (26)of the preform; wherein the mechanical device (33) includes at least onemechanical forming surface (37) which, on displacement of the preformthrough the passage, displaces material abutting against the formingsurface toward a centre axis of the passage.
 2. The method as claimed inclaim 1, wherein said defining surface (14) is reformed to asubstantially planar form.
 3. The method as claimed in claim 1, whereinthe mechanical device (33) is at a temperature exceeding the glasstransition temperature of the thermoplastic material.
 4. The method asclaimed in claim 1, wherein the mechanical forming device (30) is at atemperature exceeding the glass transition temperature of thethermoplastic material by at least 20° C.
 5. The method as claimed inclaim 1, wherein the thermoplastic material, on abutment against themechanical device (33), is heated to a temperature exceeding the maximumtemperature at which said material was adjusted before undergoingforming at which the substantially amorphous blank was reformed into thepreform.
 6. The method as claimed in claim 1, wherein, on abutmentagainst the mechanical device (33), the thermoplastic material is heatedto such elevated temperature that the thermoplastic material shrinks inthe circumferential direction.
 7. The method as claimed in claim 1,wherein the mechanical forming device (30) is at a temperature exceedingthe glass transition temperature of the thermoplastic material by atleast 40° C.
 8. The method as claimed in claim 1, wherein the mechanicalforming device (30) is at a temperature exceeding the glass transitiontemperature of the thermoplastic material by at least 60° C.
 9. Anapparatus for producing a container (10) of thermoplastic material witha mouth portion (11), a container body (12) and a bottom portion (13)from a preform (20) with a mouth portion (21), a bottom portion (23) anda preform body (22), in which the preform has been formed by expansionin the axial and/or radial direction of a hollow blank or originallysubstantially amorphous material, and in which the apparatus includes amechanical forming device (30) which, during reforming of the preforminto the container abuts against material in the preform, wherein:themechanical forming device (30) includes a forming portion (33) providedwith channels for thermal medium, and with a forming surface (37)disposed, under energy exchange with material in the preform, todisplace material shrunk by heating or the material toward the centreaxis of the preform; the forming portion (33) includes an in-feedportion (32) and the forming surface (37) for temperature conditioningof the preform (20) and wherein the in-feed portion (32) is connected toan intermediate portion (63); the intermediate portion (63) includes anafter-treatment surface (38) for supplementary release of tensionsembodied in the plastic material; and the intermediate portion (63) isconnected to a discharge portion (34) with a contact surface (39) forcooling the formed container (10).
 10. Apparatus according to claim 9wherein the discharge portion comprises an open discharge end throughwhich the formed container is discharged from the apparatus. 11.Apparatus for the production of a container (10) of thermoplasticmaterial, with a mouth portion (11), a container body (12), and a bottomportion (13) from a preform (20) with a mouth portion (21), a bottomportion (23) and a preform body (22), in which the preform has beenformed by expansion in the axial and/or radial direction of a hollowblank of originally substantially amorphous material, comprising:meansfor forming the preform; means for heating the material in the preformbody (12) by abutment against a mechanical device (33) of a mechanicalforming device (30) at elevated temperature, thereby reducing thecircumference of the preform; means for displacing at least one materialportion of the preform body for formation of the container (10), througha passage (31) wholly or partly defined by the mechanical device (33) ina direction towards the centre of the preform; means for forming anouter defining surface (14) of the container with material portionsbeing at a distance to the centre axis (16) of the container less thanthe distance of corresponding material portions to the centre axis (26)of the preform; wherein the mechanical device (33) includes at least onemechanical forming surface (37) which, on displacement of the preformthrough the passage, displaces material abutting against the formingsurface toward a centre axis of the passage.